The Crestmont serpentine patch

Every neighborhood in Oakland with a “mont” in its name has bedrock exposed. Crestmont is a good example. Most of the Crestmont neighborhood, north of Redwood Road and east of Holy Names University, sits on serpentine rockmore precisely, serpentinite. This is the biggest exposure, at the intersection of Crestmont Drive and Westfield Way. Below is an exposure just to the north (click full size), which is somewhat bluer than the blue-green of the main patch. I’ve shown pieces of this rock to local geologists, asking them “Is this serpentinite or blueschist?” They usually say “Beats me” or, even better, “Yes.” These rocks are not easily assessed by just eyeballing them.

The open lot on the east side of Crestmont near Samaria Lane displays some splendid outcrops of this “lizard-skin” stone:

10 Responses to “The Crestmont serpentine patch”

I’ve noticed when driving around a lot of the serp exposures in road cuts on small roads (around Cloverdale in particular) that there is a lot of creep and other mass wastings in this stuff, is there any particular problem in this location with such effects? I ask becuase people are footing thier houses in it.

The serpentine here is fresh rock, exposed when the land was developed, so I didn’t notice any deterioration . . . yet. On the other hand, my very first post showed deterioration of this same serpentinite.

Serpentinite weathers rapidly and is a lousy material to build a house on. If you ever drive into the mountains behind New Idria you will see roads carved into a huge body of serpentine – the worst roads I’ve ever seen in California. When it rains it’s like driving on toothpaste.

Fortunately I’ve visited New Idria (twice now) only in the dry season. But those are dirt roads. Marin County has plenty of serpentinite but its paved roads seem to be OKequally prone to slumping, I meanon all kinds of rock. Maybe they just keep up their roads well.

Todd, your first photo shows a typical blueschist from that serpentinite belt. I call it blueschist not because I’ve checked it out under the microscope but just to differentiate it from the slippery-looking serpentine rock. I have a piece from nearby that is full of holes. All I can think is that there was another mineral, perhaps talc, that has weathered away from around and inside it. The second striped stone is hard to tell from the photo. Serpentinite has great variety. I have a fat book of research papers on serpentinite that I’m partway through; we have a lot to learn still.